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Maybe a better design solution would have been to use an inverted chamber with an appropriately sized ball bearing on the bottom, rather than a bubble on top.  Paul Dolkas

A fewslight problems here Paul.  Normally, you require a transparent bubble, so you can see stars through it.  No light will travel though a ball bearing (bb).

Then you’d still need a collimating lens to focus the bb at infinity like the Sun and stars.

After that, the radius of the ball dish would have to equal the focal length of the collimating lens, so the Sun & bb kept in line without the need for fiduciary marks when the sextant was tilted in the direction of viewing.  Therefore, the dish would be very shallow.  Could you get a bb smooth enough to move freely on such a shallow dish?  A spot of dust and oops!  I’ve no doubt there’d be other problems as the experts will no doubt tell us.

With a pendulous reference sextant the graticule is frequently on a transparent disc dangling on a quartz thread suspended in a suitable fluid.  It amazes me how you can lay them on their side occasionally without damaging them (well maybe that’s why my index errors keeps changing!). DaveP